The present invention relates to a semiconductive heterojunction device which is particularly useful as a photovoltaic device such as a solar cell.
Conventional solar cells typically comprise a P-N junction formed in a monocrystalline silicon substrate. Typically, an N-type surface region is diffused into a P-type silicon substrate and ohmic contacts are applied. In operation, the device is exposed to solar radiation; and phetons, incident upon the N-type surface, travel to the junction region and the P-type substrate where they are absorbed in the production of electron-hole pairs. Holes created in the junction region (or which diffuse to the junction region) are swept by the built-in voltage to the N-type surface region where they either leave the device as photocurrent or accumulate to produce a photo-induced open circuit voltage.
The conversion efficiency of conventional solar cells, however, is seriously limited by a number of factors. One such factor is that the built-in voltage is limited by the relatively narrow bandgap of the N-type silicon and the limited extent to which both layers of silicon can be doped. While the built-in voltage of the device can be increased by increased doping of both layers forming the junction, such excess doping tends to reduce conversion efficiency of the device by reducing its collection efficiency. As a consequence, the open circuit voltages of typical silicon solar cells are only about 50 percent of the silicon bandgap.
A second factor limiting the conversion efficiency of conventional silicon solar cells is the fact that silicon tends to absorb high energy photons (photons of blue and violet light) near the surface, typically within a micron thereof. As a consequence, many of the high energy photons are absorbed near the surface of the N-type region and the carriers generated by this absorption recombine at the surface. Such recombined carriers are thereby lost as sources of photocurrent.
Yet a third limiting factor is the fact that lower energy photons (photons of red and near infra-red light) tend to penetrate deeply into silicon before they are absorbed. While minority carriers created by deep absorption can contribute to the photo current if minority carrier lifetimes are sufficient to permit them to drift into the junction region, the high temperature diffusion step required to form the N-type region significantly reduces minority carrier lifetime in the P-type substrate. As a consequence, many carriers created by deep absorption are lost to the photocurrent.